1. Field of the Invention
The present invention relates in general to the transmission of video signals and, in particular, to a system for scrambling and descrambling a composite video signal by altering the synchronization of that composite video signal.
2. Background Art
Systems for delivering numerous channels of television programming directly to consumers on a subscription basis are widespread and include such technologies as cable and direct satellite television. Each television channel broadcast over cable or direct satellite television system includes at least a composite video signal. As known in the art, the video image displayed on the cathode ray tube (CRT) of the television is in fact composed of a series of video images displayed frame by frame wherein the persistence of vision of the human viewer causes the individually displayed frames to be perceived by the viewer as a continuous image. Each frame displayed upon the CRT is created by an electron beam which begins by scanning lines upon the screen from left to right. The displayed image is built up by repeatedly scanning horizontal lines beginning at the top and working down to the bottom of the CRT in what is termed a raster pattern. In the United States televisions conforming to the NTSC standard operate by moving the beam from left to right at 15,750 lines per second and top to bottom at 60 lines per second. Each time the beam reaches either or both of the far borders (right or bottom) of the raster it must flyback to the respective one of the near borders (left or top) so the next trace of the screen can begin. During this flyback (or "retrace", as it is sometimes called) no picture information is sent to the screen which would otherwise over-write the previously scanned lines. Most importantly, the horizontal and vertical movements of the electron beam must be synchronized to insure that each of desired video frames are correctly aligned, scan line to scan line, so that the video image is stable and properly displayed.
To achieve synchronization, the composite video signal contains not only the video signal, but also blanking pulses and synchronizing signals. The blanking pulses cause the video signal amplitude go to the black level so that the retraces in scanning will not be visible. The synchronizing signals, both vertical and horizontal, indicate the time for retracing and are located within the blanking period when no picture information is transmitted.
Many of the cable and direct satellite television systems delivering television programming rely on the ability restrict individual consumer access to all or certain ones of the available channels. Restricted access is achieved by scrambling the signals at a central facility and providing a descrambling apparatus as part of a "set top box" located at each subscriber's location where each set top box is configured to descramble certain ones of the channels depending upon the service paid for by the particular subscriber.
One of the most commonly used methods of scrambling video signals in cable and satellite direct television systems is "sync suppression." By suppressing either or both of the vertical and horizontal synchronizing signals within the composite video signal the various scan lines will normally not be synchronized one to another causing rolling and drift in the picture rendering the image essentially unviewable thus deterring one from watching a television program or channel to which one has not subscribed and paid for. In this technique a "pilot signal" containing the original synchronization system is transmitted apart from the remainder of the composite signal and is used by the descrambler at the user's location to reconstruct the correct composite signal to permit the viewing of a proper video image.
It has been found that certain commercially available television sets are able to substantially synchronize a video signal without the original synchronization information being transmitted therewith. Accordingly, it is an object of the present invention to provide an scrambling apparatus and method for scrambling a video signal such that the video signal will be more difficult to recover without authorization of the cable or satellite television provider.
Another commonly used method of scrambling video signal is "random line inversion." This method reverses the polarity of random individual lines of the video signal. Recovery of the original signal is accomplished by transmitting a code with the scrambled video such that the reversed lines can be restored. This method involves some significant processing overhead as each signal must be demodulated to baseband to recover the code and descramble the video signal. Subsequent remodulation to a particular channel is then also required. As a result, implementation of the random line inversion approach to video scrambling is rather expensive. Moreover, such method of scrambling may not produce a effect upon the video image sufficiently detrimental to discourage all users from watching the scrambled channel.
Consequently, it is another object of the present invention to provide a scrambling technique which conceals the video picture to a degree which discourages unauthorized viewing of a program.
It is an associated object of the present invention to provide an apparatus and method for descrambling a signal scrambled by the apparatus and method disclosed herein.
These and other objects of the present invention will become apparent to those of ordinary skill in the art in light of the present specification, drawings and claims.